Online outrage is brewing after a 5-year-old Croatian girl named Nora Situm was reportedly told she had to pay $837,000 the day before she was to travel to the United States.

The Philadelphia Inquirer reported that Nora's mother, Dana Atanosovska Situm, held a press conference on Wednesday to thank supporters who helped raise money Nora needed to join an experimental treatment program at the Children's Hospital of Philadelphia (CHOP). The family had initially received a bill for $575,000 for the treatment process -- and then was surprised to receive another $262,000 bill for follow-up costs for the next five years, according to the paper.

"I corresponded with the hospital until 2:40 a.m. this morning," she explained. "It was a shock to see there were now two invoices. After going through this, I will never say anything against our (Croatia's socialized) health system. What they have over there (in the United States) is just terrible."

According to The Atlantic, Nora Situm suffers from acute lymphoblastic leukemia (ALL). ALL a rapid-growing cancer of the white blood cells, according to the National Marrow Donor Program. White blood cells called lymphocytes are supposed to fight off infections, but in patients with ALL, their bone marrow produces abnormal cells that can't grow into lymphocytes. They cancerous cells then take over the body and leave little room for healthy red blood cells, white blood cells and platelets.

The treatment which Nora was scheduled to enroll in is a gene therapy that was created at the Perelman School of Medicine at the University of Pennsylvania, according to the Inquirer. It works by genetically modifying the patient's T-cells from the immune system to attack the cancerous blood cells. Lead scientist Carl June previously explained that this production process alone costs about $20,000, the Inquirer reported. Other hospital procedures, tests and medications can drive the price much higher.

Angela Mattie, chair and associate professor in the department of healthcare management & organizational leadership at Quinnipiac University in Hamden, Conn., told CBSNews.com that the $837,000 bill doesn't seem out of the ordinary. In addition to the hospital costs, Mattie added that years of research, salaries and the time and effort of the PhD oncologists who are in the laboratory to develop the cure can increase the bill. There's also the price of the animal testing phase of the clinical trial, added Mattie, who has no involvement in this case.

"It takes a lot of time effort and energy to discover something that's going to make a difference, and that's the reality of the marketplace," she said, adding that the costs help ensure the U.S. as a "relatively safe and effective pharmaceutical process."

Mattie thinks that part of the outrage over the bill could be due to cultural differences. Since Croatia has a subsidized healthcare system, citizens may not be used to seeing how much these procedures can actually cost.

"From a cultural perspective, it may seem completely egregious to have this kind of bill because people in other countries may not be used to seeing the health care costs," she explained.

So far, 10 adults with chronic lymphocytic leukemia (CLL), and two children with acute lymphoblastic leukemia (ALL) have received the treatment, CHOP said. Nine of them have responded to treatment, and five entered complete remission. The Inquirer added that one child subject had a similar case to Nora's in that they both had relapsed after several rounds of chemotherapy. The 7-year-old girl has been in remission for nine months.

When news of Nora's additional bill came out, people took to CHOP's Facebook page to complain. Posts as far back as early January on unrelated topics were filled with angry comments in support of Nora, and many others wrote that they wished Nora well on her fight against cancer.

"It's a shame for the whole Country USA! Price-calculating, price increasing... it seems like on the Wall Street Exchange! (How) much should cost the little heart of Nora?" Daniel Anusic posted on CHOP's Facebook page.

"As you no shame to take that kind of money ($ 837,224) from a sick child ... how can you be so cruel????!!??????? Shame on you!!!!!! N O R A!!!!!!!!!!!" Davor Marsal Androcec wrote.

On Wednesday night Situm tried to clarify the situation on a post on the official CHOP Facebook page. She explained that she misunderstood that she had to pay both the bills immediately.

Dear all who have during these several days become part of a huge family of my Nora.

I thank you for everything that you have been doing for her through your calls and your enormous wish to help her. I believe that your comments on the Children's Hospital of Philadelphia Facebook pages were written in good faith.

However, I feel the need to tell you in this way that the reasons you believe justify such reactions are not the fault of the hospital in Philadelphia. Because of so many things that have happened within a few last days I have to inform you in good faith that there has been a big misunderstanding in the correspondence with the hospital. I have to point out that during this large-scale humanitarian action, which lasted only for a few days, many e-mail addresses have been involved so that we didn't even know whom the hospital sent a mail with the total amount needed for our Nora's medical treatment.

Therefore, I apologize that due to all this hurry an omission occured related to an unread mail concerning additional funds which was sent to the association. Therefore, I would like to ask all of you, for the sake of my child's well-being, not to write any more bad comments on the hospital pages. I am asking you this because I would not like the blame to be put on people who only want to help my child. Please have understanding for what I have written here because you really must realize how difficult it is for us this night before we leave to this institution, especially with respect to the fact that it is exactly from their doctors that we expect help.

As everything else so far, we are asking you this in the name of our Nora, but also in the name of all other children who may have tomorrow an open way to new life.

Thank you all from the bottom of our hearts!

Citing privacy laws, CHOP couldn't confirm that Nora was or is going to be a patient at their hospital or what treatment she was receiving. The hospital did confirm the above post on the CHOP Facebook page was written by Nora's mother.

The hospital also clarified its policy on international patients for CBSNews.com in a statement:

CHOP's process estimates the costs of treatment in advance and seeks payment at the time treatment begins. Additional follow-up clinical treatments are sometimes necessary and can be administered over several years, either at CHOP or back in the patient's home country. CHOP does not charge for this follow-up clinical treatment at the time of initial treatment. If the child is not further treated at CHOP, CHOP will never charge for the follow-up treatment. However, CHOP does explain those potential costs to patient families at the outset so they understand the financial issues they may be facing.

We try to ensure that all international families understand the difference between the initial costs of treatment charged by CHOP, which does not change, and the potential future costs which will depend on future clinical treatments.

According to CBS station KYW in Philadelphia, Nora landed Thursday evening at Philadelphia International Airport wearing a protective mask with two doctors and her parents. Her treatment is expected to last at least a few months.

"This is the only chance that she has. We don't know how small or big it is, but it's a chance. In Croatia there's no more chances," Diana told KYW.